A study of the expression and pathogenesis of Moloney murine leukemia virus (M-MuLV) is proposed. Experiments on expression center on tissue-specificity. In particular, elements within the long terminal repeat (LTR) will be investigated. We will insert or substiture sequences form heterologous viral and cellular genes into the M-MuLV by recombinant cloning techniques. The sequences will include enhancer, promoter, and promoter- proximal elements. The altered LTR's will be tested for expression in a variety of differentiated cell types. In addition, the altered LTR's will be reconstituted into replication- component M-MuLV and tested for tissue-specific infection in vitro and in vivo. These experiments may provide important information on tissue-specific transcription, and they may also be important in the design of retroviral vectors. Experiments on M-MuLV pathogenesis take advantage of the fact that several M-MuLV's with altered LTR's show altered pathogenic potential. This ranges from non-pathogenic M-MuLV's (Mo+PyF101 M-MuLV) to M-MuLV's with altered disease spectrum (PyF101+Mo M-MuLV). These viruses provide a powerful tool for investigating the nature of leukemic and preleukemic changes induced by M-MuLV. Hematopoietic tissue from preleukemic mice inoculated with wild-type M-MuLV will be characterized for preleukemic changes. Hyperplasia will be documented and characterized by flow cytometry and biological stem cell assays. The viral status of preleukemic cells will also be investigated. Mice inoculated with the variant viruses will also be examined for preleukemic charges. The variant viruses may identify both critical and irrelevant preleukemic changes. End-stage tumors induced by wild-type and variant M-MuLV's will also be studied. The cell types of the tumors will be determined, and the nature of the viruses in the tumors will be determined. The tumors will also be examined for proviral insertion in the vicinity of cellular proto-oncogenes. These studies may identify critical steps in M- MuLV induced leukemogenesis.